There are plenty of study needed before extensive utilization of biochar in vegetable production systems. As the findings in the thesis were concluded from pot experiments, the effect of biochar on nitrogen processes should be assessed in the special field for several growing seasons. The additional method of biochar is the fields where studies are urgently needed now.
6.4.1 How to determine whether the soil should be added biochar?
It is important to define the role of biochar before answer this question. Biochar was firstly studied as a carbon-rich product for fixing atmospheric carbon in soil system.
When biochar is used as a carbon storage material, it can be returned to any soils in natural systems.
In recent years, increasing attentions have paid with regard the amending capacities of biochar (Seen detail in 1.5 Chapter 1). The characteristics of biochar produced from different original materials or pyrolysis conditions are different attributing to various amending functions. When biochar is treated as a soil amendment, its proper producing condition is the key to determine its effect on soil problem. Furthermore, biochar may not as effective as other specific amendments for certain soil problem (such as heavy metal pollution) and lots of biochar will be needed when biochar is the only amendment used for that problem. The expense, efficiency and the potential negative effect of biochar should be considered for determining whether biochar is the optimal choice.
However, the problems in most cultivated soils are various and correlative and biochar addition could settle several soil problems at once. For example, high leaching loss, low nitrogen retention are the major two constraints for plant uptake nitrogen in this study.
A significant promotion of plant nitrogen uptake was found in treatments with biochar addition in continuous growing seasons, which was contributed to the comprehensive effects of biochar on soil nitrogen retention, leaching loss and even soil pH maintaining (6.2 in Chapter 6). It was economical and environmentally friendly to use biochar as the soil amendment.
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In our opinion, there is no need to add biochar to all kinds of fields. There are some questions that we suggest to ask before adding biochar to soil listed as below:
What is biochar used as?
A soil amendment A carbon storage material Other roles (1) What is the problem
about the soil?
(1) Where is the biochar from?
(1) Why choose biochar for it?
(2) Can the problem be alleviated / amended by biochar?
(2) Is the producing technique low- carbon?
(2) What advantages does
biochar have
compared with
previous methods?
(3) How much biochar should be added for the problem?
(3) Will the system be affected by biochar addition?
…
(4) Is there any negative effect or risks along with biochar addition rate?
…
(5) Is it economical to use biochar as the amendment for this problem?
…
6.4.2 How to determine the optimal biochar addition rate and frequency?
When biochar treated as an amendment, its addition rate should be determined by the amending function of certain problem in theory. Unfortunately, the effect of biochar addition on problem amending is not commonly linear growth. For example, two addition rates (1% and 5% biochar) were used in this study. Soil leaching loss was further reduced by more biochar addition but no significant effect was found on soil
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mineral nitrogen content, total nitrogen loss, soil pH value and plant nitrogen uptake between 1% and 5% biochar addition. Furthermore, Kammann et al. (2012) found quinoa growth was retarded with a high biochar addition and Feng et al. (2017) reported a significant increase in ammonia volatilization in paddy soil with 3% biochar addition.
However, meta-analysis of biochar addition rates provided little insight into how biochar should best be applied (Biederman and Harpole, 2013).
Furthermore, the frequency of biochar addition is another problems in practical application. For the field agricultural residue returned as the form of biochar, it will be added after every growing season. The rate of biochar addition may be low for one time but the rate of cumulated addition will be high and the long-term effects of multiple additions are still lack of study.
In this study, it was found soil pH buffering capacity could be used to determine biochar addition rates by using the linear relationship between slope of titration curve and biochar addition rate. Additionally, biochar should be added again when soil pH values decrease and no significant difference was shown in the pH of the soil without the addition of biochar. Findings in Chapter 4 suggest that multiple additions of biochar are necessary to prevent acidification.
Therefore, the proper biochar addition rate should be in the range of high efficiency rather than best effect on the aiming problem. The indicators related the aiming problem could be used to determine the time for adding biochar again. Moreover, the long-term effects on other soil physical / chemical characteristics and the response of plant to biochar addition need to be taken account and also studied further.
6.4.3 Use biochar as a carrier for nitrogen transport
The availability of mineral nitrogen retained by biochar has been demonstrated in this study. In addition, Taghizadeh-Toosi et al. (2012) has reported that the ammonia adsorbed on biochar could be utilized by plants again, thus implying that biochar could be used as a carrier for transporting nitrogen.
Non-point pollution is a serious problem in China and nitrogen from agricultural fields contributes approximately 40% of the nitrogen in the water system (Yang et al.,